The present invention relates to a semiconductor laser of the surface emitting type and, in particular, relates to a vertical cavity surface emitting laser (VCSEL) that is appropriately applicable to general use such as consumer applications.
Since a vertical cavity surface emitting laser emits a laser beam from the surface of its substrate in a direction perpendicular to the surface, two-dimensional parallel integration is possible. In addition, since the divergence of the beam is relatively narrow, having an angle of the order of 10 degrees, the vertical cavity surface emitting laser is characterized in that coupling with an optical fiber can be implemented with ease and the device can be inspected easily. For this reason, the vertical cavity surface emitting laser is developed aggressively as a device appropriate for creating an optical transmitter-receiver module (an optical interconnection module) of the parallel communication type. Thus far, application targets of the optical interconnection module have included parallel connection between circuit boards and between boxes of equipment such as computers and short-distance communication through an optical fiber. It is expected, applications of the optical interconnection module in the future will include a large-scale computer network and a telecommunication network.
In general, the vertical cavity surface emitting laser has a configuration of a cavity comprising an active layer made of GaAs and GaInAs sandwiched by a mirror above the active layer and an underlayer mirror on the substrate side beneath the active layer. In comparison with an edge emitter semiconductor laser, the length of the cavity is extremely short. It is thus necessary to make laser oscillation easy to generate by setting the reflectivity of each of the mirrors at a very high value of equal to or greater than 99%. For this reason, a distributed Bragg reflector (DBR) is normally used as a mirror. The distributed Bragg reflector is created from layers of low-refractivity materials made of a AlAs and layers of high-refractivity materials made of GaAs. The low-refractivity and high-refractivity materials are stacked on each other alternately with a period of 1/4 of the wavelength.
Since the reflectivity can be increased by increasing the number of pairs in the distributed Bragg reflector, in many cases, 30 to 40 pairs are used. If a large number of pairs are used in the distributed Bragg reflector as such, however, it becomes difficult to manufacture the distributed Bragg reflector and, moreover, the yield of the device deteriorates. In addition, a series resistance increases, giving rise to a problem that the power consumption also increases as well. The height of the vertical cavity surface emitting laser also increases. As a result, electrical wiring becomes difficult to implement, making it hard to integrate the vertical cavity surface emitting laser with another semiconductor device such as a transistor for driving the laser. Because of such problems, it is desirable to reduce the number of pairs in the distributed Bragg reflector to as small a value as possible. The number of pairs in the distributed Bragg reflector can be reduced by increasing the difference in refractivity between the low-refractivity layer and the high-refractivity layer.
It is thus important to select a material with a great refractivity difference. From a standpoint of suppressing dislocation generation, however, it is necessary to select a material which exhibits a property of lattice-matching the substrate. At the present time, there are only few materials that satisfy the requirements in both the aspects. For example, the GaAs substrate is a substrate material which allows a crystal having good characteristics to be obtained with ease and, in addition, a semiconductor laser created on such a substrate exhibits a stable temperature characteristic. Therefore, the GaAs substrate is widely used in general applications. At the present time, however, materials which exhibit a lattice-matching property are AlAs and GaAs, materials having a low refractivity and a high refractivity, respectively.
Recently, however, an invention has been disclosed in Japanese Patent Laid-open (Kokai) No. Hei 6-132605. A lattice-mismatch problem is relieved by providing a buffer layer between the substrate and the mirror in a vertical cavity surface emitting layer using lattice-mismatched semiconductor materials. In this example, AlInP is used as a low-refractivity material whereas InGaAsP is used as a high-refractivity material. Both the materials do not exhibit a property of lattice-matching the substrate. However, both the refractivity materials have a fixed thickness equal to one-fourth of the wavelength, a thickness which is much greater than a critical layer thickness of about 10 nm. As a result, effects of lattice mismatching can not be avoided, giving rise to a problem that crystal defects are generated easily.
The number of problems having to do with the number of pairs is particularly high for laser wavelengths in the 1.3 .mu.m and the 1.55 .mu.m ranges. InP is mainly used as a substrate and InGaAsP is used as an active layer in the case of a laser with a long wavelength in such ranges. InP used as a substrate has a large lattice constant and a large refractivity difference can not be obtained in the reflector material in order to lattice-match the substrate. It is thus necessary to increase the number of pairs to 40 or even greater. On the other hand, the semiconductor laser created on the InP substrate has another problem that the characteristics thereof greatly change with temperature. For this reason, it is necessary to use the semiconductor laser by adding a device for stabilizing the temperature, giving rise to difficulties in providing the semiconductor laser for general use such as consumer applications. The problem relating to the number of pairs and the temperature-characteristic problem described above make it difficult to put the long-wavelength vertical cavity surface emitting laser to practical use.